1. Field of the Invention
This invention relates to formation of integrated circuit structures. More particularly, this invention relates to an etch process for removal of materials remaining from the formation of cobalt silicide on an integrated circuit structure on a semiconductor substrate in preference to the removal of cobalt silicide.
2. Description of the Related Art
In the formation of integrated circuit structures, and particularly in the formation of MOS devices using polysilicon gate electrodes, it has become the practice to provide a metal silicide layer over the polysilicon gate electrode, and over the source/drain regions of the silicon substrate, to facilitate electrically and metallurgically connecting the silicon to metal interconnects. Thus, for example, a metal layer capable of reacting with silicon to form a conductive metal silicide is usually blanket deposited over the polysilicon gate electrode and the source/drain regions of the silicon substrate, as well as over the silicon oxide insulation regions of the substrate, e.g., the field oxide regions. The structure is then heated sufficiently to cause the metal in contact with the silicon to react, thereby forming the desired metal silicide.
While titanium silicide has been widely used in the past, as the dimensions of the integrated circuit structure have become smaller, the use of cobalt silicide (CoSi.sub.2) is becoming preferred over titanium silicide because of certain problems encountered in the use of titanium silicide in the formation of narrow lines. However, cobalt does not reduce oxide as well as titanium, and thus the cleanliness of the semiconductor substrate is much more critical if cobalt is used to form the desired metal silicide.
It has been proposed in the past to form a capping layer of titanium metal or titanium nitride over the cobalt layer prior to formation of the cobalt silicide to avoid some of these problems.
More recently, in copending patent application Ser. No. 08/833,597, pending, entitled "PROCESS FOR FORMING IMPROVED COBALT SILICIDE LAYER ON INTEGRATED CIRCUIT STRUCTURE USING TWO CAPPING LAYERS", which was filed by us with another inventor on Apr. 7, 1997, assigned to the assignee of the assignee of this invention, and the subject matter of which is hereby incorporated by reference, we proposed to form a dual capping layer of titanium metal and titanium nitride over a cobalt layer prior to silicide formation to thereby form a colbalt silicide layer of more even thickness. Since it is very desirable to form a cobalt silicide layer of uniform thickness for purposes of subsequent implantation, the use of such a dual capping structure to provide such a uniform thickness cobalt silicide layer is also desirable. However, the use of such a dual capping layer, in turn, requires the removal of a number of different materials after the cobalt silicide layer is formed, including unreacted cobalt, titanium, and titanium nitride, as well as any other reaction products of cobalt other than the desired cobalt silicide layer. Such selective removal, in turn, requires an etch system which is capable of removing all such materials while being highly selective to cobalt silicide, i.e., which will remove such materials as unreacted cobalt or other cobalt reaction products, as well as titanium and titanium nitride, without removing any substantial amount of cobalt silicide.
It would, thereby, be desirable to provide an etch system which will remove unreacted cobalt, and titanium and/or titanium nitride, as well as any other reaction products of cobalt other than the desired cobalt silicide layer, while exhibiting high selectivity with respect to cobalt silicide.